Contents

Abstract

The paper describes the paleomagnetic studies of
gabbronorites of the southern White Sea region. It has been found
that the major part of the rocks studied was remagnetized during
the Svecofennian tectonomagmatic epoch and that the secondary
magnetization is fairly stable. Its direction deduced from the
intrusions on the Borshevets Island ( N = 27,
D = 6.1o,
I = 48.3o,
k = 50.6,
a95 = 3.9 )
corresponds to the
coordinates of the pole (54.8o N, 206.8oE,
dp = 3.4o,
dm = 5.1o ), which is in a good agreement with the
"Svecofennian"
fragment of the apparent polar wander path of Fennoscandia. This
means that the age of this magnetization is about 1.8 Ga. The
paleomagnetic pole deduced from the contact zone of the intrusion
on the Emestrov Island lies close to the group of early Karelian
poles. However, judging by its statistical characteristics, the
hypothesis on the primary origin of the magnetization of this
intrusion should be regarded only as tentative. The rocks of the
central part of this intrusion and also of the intrusion located
near the town of Belomorsk were remagnetized during the Svecofennian
period as well. It is concluded that the narrow spectrum of
unblocking temperatures suggests that the superimposed
magnetization has a partial thermoremanent nature, and the
temperatures of the secondary heating of the rocks under
investigation are estimated.

1. Introduction

The research efforts aimed at construction of the apparent polar
wander path (APWP) for the Fennoscandian Precambrian started more
that 30 years ago
[Katseblin, 1968;
Neuvonen, 1965].
These and more recent works resulted in several
versions of the APWP. To analyze the history of the Fennoscandian
Shield drift in the Early Proterozoic, the most recent version of
the APWP is typically used
[Elming et al., 1993].
Not all
fragments of the Early Proterozoic track of this curve are well
justified;
for instance, the fragment between the "key" poles with
the ages of 1.88 Ga and 1.76 Ga is built on the basis of a sufficiently
large number of reliable paleomagnetic determinations. There is
also a group of reliably dated poles with the ages of about 2.4 Ga.
At the same time, the part of the curve between 2.4 Ga and 1.88 Ga
is poorly justified.

In addition, greatly differing amounts of the paleomagnetic data
are available for different parts of the shield. There are more
than 200 paleomagnetic determinations for its western part (the
territory of Finland and Sweden), while for the Karelian-Kola
region, there are less than 40 determinations, and until recently,
only 14 of them have been for the early Karelian (2.5-1.9 Ga). The
latest paleomagnetic studies
[Damm et al., 1997;
Khramov et al., 1997]
have improved the situation only a little.
Specifically, the southern White Sea region still remains poorly
studied from the paleomagnetic point of view. The goal of this
paper is to fill the gap to some extent.

2. Geology and Sampling

The southern White Sea region is a part of the
White-Sea mobile
belt separating the Karelian granite-greenschist and Kola
granulite-gneiss regions. The determining role in the structure of
the southern White Sea region is played by the Archean complex of
homogeneous rocks of the tonalite-plagiomicrocline granite
composition which has experienced multiple structural and
metamorphic transformations. A characteristic feature of the
southern White Sea region distinguishing it from the western White
Sea region is a small amount of mafic rocks. The studies of this
region, including the structural-metamorphic analysis have revealed
three groups of mafic rocks, the most ancient among which (2.8 Ga)
are analogs of volcanic rocks of the Archean greenschist belts of
Karelia. According to the isotope-geochemical data the remaining
groups of mafic rocks (to which gabbronorite intrusions belong
(Figure 1)) are considered to be of one age and formed in the
interval 2.5-2.45 Ga
[Chekulaev et al., 1994].

The structural scheme of the region of the southern White Sea is
completely determined by late Svecofennian deformations. Relicts of
the earlier structures are present now only in some lens-like areas. A
detailed mapping of these areas made it possible to reveal three
long stages of the endogenic development: two Archean and one
Proterozoic-Svecofennian.

Figure 1

All mafic rocks of the southern White Sea region have experienced
the Svecofennian metamorphism under the conditions of epidote
amphibolite-amphibolite facies of increased pressures
( T = 650o-550 oC,
P = 7-8 kbar)
[Kotova, 1988].
As a
result of intense transformations, the most ancient mafic rocks
have been preserved mainly as relics, which makes the choice of
oriented specimens for paleomagnetic studies difficult. For this
reason the objects for our studies were chosen to be gabbronorites
(taken from intrusions
B and
V on the Borshevets Island, intrusion
D on the Emestrov Island, and intrusion
I near Belomorsk) and
granitoids of the Yukovo complex (Figure 1).

Oriented hand samples of gabbronorites were collected from
(1) a 1-m-thin dike
B sampled at the distance of 50 m (13 hand
samples, including 4 samples of gneisses cut by the dike);
(2) about 250-m-thick intrusion
V (31 hand samples),
(3) intrusion
D with the visible thickness of 70 m and sampled length
of 145 m (27 hand samples) (these rocks had different granularities
(from small-grain rocks at the intrusion margins to large-grain
and gigantic-grain rocks at its center)),
and (4) intrusion
I with the visible thickness of about 50 m (23 hand
samples).

From the body of granitoids near the village of Yukovo, 18 samples were
collected along the White Sea coast at a distance of about 1.5 km.
In spite of metamorphic Svecofennian transformations, both mafic
rocks and granites had regions of rocks with the well-preserved
magmatic structures.
The samples were oriented by a magnetic compass.

3. Instrumentation and Experimental Procedure

Oriented hand samples were cut into cubes with the rib of 2 cm.
From each hand sample, from two to eight cubic specimens were prepared.

The remanent magnetization of the specimens was measured by JR4
"Geophysica" spinner magnetometers (Brno, Czech Republic) at the
paleomagnetic laboratory of the All-Russian Petroleum Research
Geological Exploration Institute at St. Petersburg. A stepwise
thermal demagnetization was performed by a setup developed at the
same Institute. The local geomagnetic field was screened in the
setup by a three-layer
m
metal screen. A part of the specimens
(intrusion
D ) was studied at the Laboratory of Magnetic Properties
of the Institute of Terrestrial Magnetism, Ionosphere, and Radio
Wave Propagation at St. Petersburg. The experiments involving the
stepwise demagnetization with an alternating magnetic field were
carried out at the laboratories of the National Geological Institute
and the Institute of Terrestrial Magnetism, Ionosphere, and Radio
Wave Propagation at St. Petersburg. The differential thermomagnetic
analysis of the specimens was performed at Kazan State University.

The obtained data were statistically processed using a standard
procedure
[Fisher, 1953;
Kirschvink, 1980;
Zijderveld, 1967]
by the IAPD computer program
[Torsvik,1986].

4. Paleomagnetic Analysis

Figure 2

The natural remanent magnetization (NRM) of gabbronorites of dike
B (the Borshevets Island) was found to range from 0.06
to 1.3 mA m-1,
and the magnetic susceptibility ( k ) was
(4.3 -5.6) 10-4 SI units. The
rocks of intrusion
B (which outcrop on the same island) proved to be more magnetic: NRM
was from 0.5 to 2.5 mA m-1,
and the magnetic susceptibility was
(4.5-6.5) 10-4 SI. The differential
thermomagnetic analysis
of the specimens from these bodies has shown that the main
magnetic
mineral in them is magnetite. Pyrrhotite is also present in some
specimens. A stepwise thermal demagnetization of specimens from
intrusions
B and
V

Figure 3

(Figures 2 and 3, respectively) has revealed the
presence of one stable component with the unblocking temperatures
TUB = 450o-530 o C. This component
is also well isolated at
demagnetization by an alternating magnetic field. The distribution
of the component directions is shown in Figure 6a; the mean
paleomagnetic direction and its statistical characteristics are
listed in Table 1.

Figure 4

The magnetic properties of gabbronorites from intrusion
D (the
Emestrov Island) were as follows: NRM
= (1.5-9.6) mA m-1,
k =(1.8-5.0)
10-4. The example of demagnetization with an
alternating magnetic field is given in Figure 4. It was found that
the characteristic magnetization components have different
directions in various parts of the intrusion. The rocks of the
marginal (contact) part exhibit a stable magnetization whose
direction distribution is shown in Figure 6c. Its mean direction
and statistical characteristics are given in Table 1. The central
part of the body is characterized by another direction (see Table 1
and Figure 6d).

Figure 5

Figure 6

The increase of
Q values from the center ( Q = 1.7 ) to the
contact zone ( Q = 2.4 ) manifests initially a magmatic origin of the
minerals, which are a bearer of the magnetization. In the near-contact
region these minerals might have saved the initial magnetization,
whereas the less magnetically tough minerals
of the
central part
of the
intrusion
had more chances to obtain a new magnetization in the
Svecofennian activization period. Actually, the most ancient
component is present exclusively in the endocontact zone, and the
rocks of the central part of the intrusion keep the Svecofennian
magnetization (Figures 6c and 6d).

The magnetization of the most metamorphosed parts of intrusion
I (Belomorsk) proved to be rather low: NRM = (0.04-0.96) mA m-1.
However, several specimens from the weakly altered parts of the
body were found to be strongly magnetic: NRM = (1.2-3.4) mA m
-1. The
magnetic susceptibility of the rocks of intrusion
I proved to be in
approximately the same limits as that of other bodies studied:
k = (4.5-5.5)
10-4. The thermal demagnetization of
these rocks revealed a lower stability of the characteristic
magnetization compared with those of intrusions
B and
V. It falls
sharply even at the first temperature steps (Figure 5) and
sometimes becomes comparable with the noise level after heating up
to 300o-400o C. The reliable (statistically significant)
directions are presented in Figure 6b. Of particular interest is
the presence of two polarities (though one of them is indicated in
Figure 6b
by only one point). The mean direction of the
characteristic magnetization of intrusion
I is shown in Table 1.

A considerable part of the specimens studied was found to have a
low-temperature component
( TUB = 200o-300 oC)

and a low-coercive
component with greatly "scattered" directions (Figure 6e). The
characteristic magnetization of granitoids of the Yukovo massif
also proved to have greatly differing directions (Figure 6f).
Because of the poor statistics of the directions, these
magnetizations are not given in Table 1.

5. Discussion

The history of rock metamorphic transformations in this region
manifests that the most probable epochs of remagnetization might
have been metamorphism episodes with the age of 2.45, 1.9, and 1.8
milliard
years.

Figure 7

Figure 7 shows paleomagnetic poles for gabbronorites of the
southern White Sea region (Table 1) and also a fragment of the
apparent polar wander path (APWP) for Fennoscandia
[Elming et al.,1993].
Note that the fragment of the APWP between the key
poles at 1.88 Ga and 1.76 Ga (which corresponds to the Svecofennian
epoch) is fairly reliable and is based on a large number of
reliable paleomagnetic determinations. The key pole with the age of
2.45 Ga is also justified statistically fairly well. At the same
time, the part of the curve between 2.45 Ga and 1.88 Ga is poorly
justified. Moreover, as recent studies
[Pisarevsky and Sokolov,1999]
have shown, there is reason to believe that it can be
reconsidered.

As can be seen from Figure 7, the most reliably defined pole
derived from intrusions
B and
V (pole 1, Figure 7) is in a good
agreement with the Svecofennian fragment of the APWP. This leads to
the conclusion that the gabbronorites of intrusions
B and
V were
fully remagnetized in the Svecofennian epoch, to be more exact,
about 1.8 Ga. This magnetization is characterized by a rather
narrow interval of unblocking temperatures (450o-530o C). It
can
be supposed that the secondary heating, which caused remagnetization,
was characterized
by the temperatures not exceeding the above indicated one.
Since the temperature of the Svecofennian metamorphism is evaluated
by petrologists within the 650o-550o C interval, the age of
this
magnetization cannot be more than 1.8 milliard
years.
However, additional
studies are needed to confirm this conclusion.

The paleomagnetic pole deduced from intrusion
I (pole 4) and both
poles determined from intrusion
D are also consistent with the
APWP. However, because of a lower quality of the determinations and
the necessity to revise the 2.45-1.88 Ga
fragment of the APWP, it
is impossible to date exactly these magnetization components. Our
tentative estimates of their ages are listed in Table 1. It is
clear that only pole 3 can correspond to the rock formation time
(2.5-2.45 Ga) because it is derived from the contact region of
intrusion
D. Intrusion
I and the central part of intrusion
D were
also remagnetized in the Svecofennian. Because of the insufficient
reliability of the 2.45-1.88 Ga fragment of the APWP, only the
lower limit of the age of this remagnetization can be estimated
(1.88 Ga).

6. Conclusions

The paleomagnetic studies of gabbronorites of the southern White
Sea region have revealed that their major part was remagnetized in
the Svecofennian epoch. The paleomagnetic pole inferred from
intrusions
B and
V on the Borshevets Island has satisfactory
statistical characteristics and is fairly reliable. By comparing it
with the APWP for Fennoscandia, its age is estimated to be 1.8 Ga.
The paleomagnetic age inferred from the contact part of intrusion
D on the Emestrov Island probably corresponds to the rock formation
time (2.5-2.45 Ga);
however, it is poorly justified statistically
and requires verification. Intrusion
I (Belomorsk) and the central
part of intrusion
D were also remagnetized in the Svecofennian
epoch; however,
the age of this remagnetization is not less than
1.88 Ga.

The narrow spectrum of unblocking temperatures of the characteristic
component of magnetization of intrusions
B and
V suggests that the
temperatures of secondary heating, which led to remagnetization of
rocks in the Svecofennian, were between 450o C and 530o C.

Acknowledgments

The work was supported by the Russian Foundation
for Basic Research (project 97-05-64113).

References

Chekulayev, V. P., O. A. Levchenkov, S. B. Lobach-Zhuchenko, and S.
A. Sergeyev, New data for the determination of the age limits of
formation of the Archean complex of Karelia, in
General Problems and Principles of Subdivision of the Precambrian (in Russian),
edited by
V. A. Glebovitskiy, pp. 69-86, Nauka, St. Petersburg,
Russia, 1994.